Controlling the Cathodic Potential of KVPO₄F through Oxygen Substitution

Exploring and tailoring new high energy density positive electrode materials is still a challenge for alkali-ion batteries. In this work, we synthesized the mixed anion phases KVPO₄F_1–yO_y (y = 0, 0.25, 0.5, 0.75, 1) and determined their crystallographic and electronic structures by combining synchrotron X-ray diffraction, X-ray absorption spectroscopy at the vanadium K edge, and ³¹P MAS NMR coupled with density functional theory calculations. These experiments confirmed that the substitution of F^– for O^2– anions occurs as a solid solution across the whole composition domain. The local environments of vanadium ions are complex and diverse since the cis and trans octahedra undergo different distortions in the presence of a vanadyl bond. The simultaneous existence of ionic V^III–F bonds and covalent (V^IVO)²⁺ vanadyl type entities is strongly affecting the electrochemical properties and potassium deinsertion/insertion mechanisms upon cycling. Ultimately, KVPO₄F_0.5O_0.5 appears as a promising positive electrode material due to its high capacity (105 mAh·g^–1), working potential (4.2 V vs K⁺/K), and sloping electrochemical curve.